Measuring, recording, and controlling apparatus



July 8, 1947. J. A. CALDWELL MEASURING, RECORDING, AND CONTROLLING APPARATUS ll Sheets-Sheet 1 Filed Dec. 1, 1941 AT RNEY INVENTOR. JOHN A. CALDWELL I45 has use FIG. I.

; MOTOR omv: P p 88 July's, 1947. J CALDWELL 2,423,479

MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 2 FIG.2. FIG.3. FIG.4. FIGS, Fl 6.

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JOHN A. CALDWELL BY g6 AT NEY y 1947- J. A. CALDWELL 9 MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 3 July 8, 1947.

J. A. CALDWELL MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 4 FIG. IO.

INVENTOR. JOHN -A. CALDWELL BY F ORNEY y 3, 1947' J. A. CALDWELL 2,

MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 1i sheets-skeet 5 FIG. I I.

INVENTOR.

JOHN A. CALDWELL A RNEY y 1947- A J. A. CALDWELL 2,423,479

MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 6 FIG. l3.

INVENTOR.

JOHN A. CALDWELL BYflW A ORNEY July 8, 1947. J CALDWELL 2,423,479

MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 7 INVENTOR.

JOHN A. CALDWELL BY ATTO NEY July 8, 1947.

J. A. CALDWELL ,423,479

MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet 8 534 FIG.|5. 533

INVENTOR.

JOHN A. CALDWELL AT RNEY A FIGJS. 50o

July 8, 1947. J. A. CALDWELL MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 11 Sheets-Sheet 10 FIG.32.

3 I f I 628 254 253 622 36 5 630 INVENTOR. 258 JOHN A. CALDWELL July 8, 1947- J. A. CALDWELL MEASURING, RECORDING, AND CONTROLLING APPARATUS Filed Dec. 1, 1941 ll Sheets-Sheet l-1 INVENTOR.

JOHN A. CALDWELL BY AT RNEY- Patented July 8, 1947 MEASURING, RECORDING, AND CONTROLLING APPARATUS John A. Caldwell, Philadelphia, Pa., assignor to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application December 1, 1941, Serial No. 421,175

43 Claims. (Cl. 234-) This invention relates to a measuring and/or control apparatus for measuring and/or controlling the value of a condition.

More specifically, this invention is directed to a self-balancing potentiometer measuring apparatus, wherein the unbalanced D. C. current of the potentiometer is transformed into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer circuit, wherein the pulsating current is amplified to operate a reversible electric motor in one direction or the other, wherein operation of the reversible electric motor rebalances the potentiometer circuit and wherein the electric motor operates indicating, recording and/or control mechanism. The potentiometer apparatus may measur a condition and operate control mechanism to regulate that condition or some other condition.

A prime object of this invention is to provide a new and novel measuring and/or control apparatus for accomplishing in an improved manner measuring, indicating, recording and control functions. By reason of this invention improved results in measuring, indicating, recording and control are obtained which cannot be obtained by present known measuring and/or control devices. These improved results are accomplished by the coaction of new and novel features which provide more accurate and faster measuring, indicating, recording and controlling operations together with long life of the apparatus. These new and novel features and the coaction thereof also form objects of this invention.

More particularly, an object of this invention is to provide a new and novel self-balancing measuring instrument having improved means for operating the balancing means.

Another object of this invention is to provide a new and novel measuring instrument having improved means for exhibiting the value of the condition being measured.

A further object of this invention is to provide a new and novel measuring instrument having improved means for controlling the value of a condition.

Still another object of this invention is to provide a new and novel self-balancing measuring,

apparatus having improved means for operating the balancing means, for exhibiting the value of the condition being measured and for controlling the value of a condition.

A further object of this invention is to provide an improved measuring and/or control instrument having the various parts thereof so located A and related with respect to each other that the various parts are easily accessible and cooperate in the simplest manner.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawing in which:

Fig. 1 is a schematic illustration of the measuring and control apparatus;

Figs. 2, 3, 4, 5 and 6 are curves showing voltages and currents present in different parts of the measuring and control apparatus under various operating conditions;

Fig. 7 is a front elevational view of the instrument with the door closed;

Fig. 8 is a side elevational view of the instrument with parts of the instrument case shown in section;

Fig. 9 is a front elevational view of the instrument with the door swung open;

Fig. 10 is a front elevational view of the instrument with parts broken away and with the door, pointer and chart plate removed showing th front of the swinging frame;

Fig. 11 is an elevational view of the back of the swinging frame with parts broken away;

Fig. 12 is a view of the back of the upper panel of the instrument showing the control point adjusting means of the control mechanism;

Fig. 13 is a front elevational view of the instrument with the door and swinging frame removed showing the interior of the instrument case;

Fig. 14 is a horizontal sectional view through th swinging frame taken substantially on the line I4-|4 of Figs. 10 and 11;

Fig. 15 is an elevational view of the slide-wire assembly with th cover thereof removed;

Fig. 16 is a vertical sectional view of the slidewire assembly;

Fig. 16A is a detail sectional view of the cable drum of the slide-wire assembly taken substantially along the line I6A-|6A of Fig. 16;

Fig. 1'7 is a bottom view of the slide-Wire assembly with the cover thereof removed;

Figs. 18, 19, 20 and 21 are diagrammatic views showing the manner in which the slide-wire and collector are secured to the slide-Wire assembly;

Fig. 22 is a schematic view of the input transformer;

Figs. 23 and 24 are elevational views of the vibrator;

Fig. 25 is a sectional View taken substantially on the line 25-25 of Figs. 11 and 12;

Fig. 26 is a horizontal plan view lookin down on Figs 1 ,12 and 25;

Fig. 27 is a detail sectional view of the cold junction housing;

Figs. 28 and 29 are elevational views of the Clamping means for securing the instrument to a panel;

Fig. 30 is a front view of the latch for swinging frame;

31 is a horizontal sectional view of latch for the swinging frame;

Figs. 32 and 33 are elevational views of stop for the door;

Fig. 34 is an elevational view of the back of the swinging frame with parts broken away showing a modification for completely operating the recording apparatus and/or the control appartus through only a portion of the total range of the instrument;

Fig, 35 is a sectional view taken substantially on the line 3535 of Figure 34; and

Fig. 36 is a front view of the apparatus shown in Fi s. 34 and 35.

Referring now to Figure 1 there is schematically illustrated the measuring and control apparatus in the form of a self-balancing potentiometer controller for measuring, indicating, recording and controlling the value of a condition, for example, the temperature value in a furnace iii. The furnace 50 may be heated by a burner H controlled by a pneumatic valve l2.

A thermocouple l3 responsive to the temperature within the furnace ill operates in conjunction with a potentiometer network generally designated at E5 and a slide-wire assembly generally designated at l6 to form a self-balancing potentiometer system. Upon a change in temperature within the furnace I!) an unbalanced D. C. current in one direction or the opposite direction is produced dependin upon the sense of unbalance of the potentiometer system. This D. C. current of one direction or of opposite direction is supplied to an amplifier unit I! having a vibrator or equivalent device generally designated at 18, a transformer generally designated at 19, amplifying vacuum tubes generally designated at 20 and 2 l, and motor drive vacuum tubes generally designated at 22 and 23.

The direct current supplied to the amplifier l! is converted by the vibrator l8 into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer system. This pulsating current is detected and amplified by the transformer l9 and is further amplified by the amplifying vacuum tubes 20 and 2!. The output from the amplifying vacuum tubes 20 and 2| is supplied to re motor drive vacuum tubes 22 and 23 for operating in one direction or another a reversible electric motor generally designated at 24. The reversible motor 24 operates a drive mechanism generally designated at 25. The drive mechanism operates the slide-wire assembly It to rebalance the potentiometer system and also operates indicating and recording means for indicating and recording the temperature conditions existing within the furnace Ill. The drive mechanism 25 also operates a controller generally designated at 25 which may take the form of a pneumatic controller such as illustrated in the C. B. Moore Patent 2,125,081, granted July 26, 1938. This controller 26 operates to control the valve 12 to maintain the temperature within the furnace It at the desired normal value.

The potentiometer network 15 may comprise the the

the

three resistances 30, 3| and 32 connected in series. These resistances are preferably formed of a manganin wire having substantially no temperature resistance coefiicient and are used for calibration purposes. Connected in parallel with the resistances 38, Si and 32 are a battery 33 which may be he form of a dry cell and a dual Vernier rheostat comprising resistances 34 and 35 and electrically connected sliding contacts 35 and 3'! associated respectively wi .1 the resistances 34 and 35. The dual Vernier rheostat may be operated by a knob 38, there being a direct mechanical connection between the knob and the contact 3-8 and a lost motion connection between the knob and the contact 31. Upon initial move ment of the knob the contact 33 is first moved and then the contact 31 is moved thereby providing Vernier adjustment. The battery 33 and the dual Vernier rheostat are connected in series with respect to each other. The resistances 34 and 35 of the Vernier rheostat may be made of copper or other material having a positive temperature resistance coefficient for compensating for temperature chan es in the battery 33. Also connected in parallel with the resistances 33, 3| and 32 are two resistances 39 and 40 which in turn are connected in series with respect to each other. The resistance 39 is preferably made of copper or other material havin a positive temperature resistance coeiiicient and the resistance 49 may be formed of manganin having substantially no temperature resistance coefficient. The resistance 39 operates to compensate for temperature changes at the cold junction of the thermocouple, it being so connected into the network that the voltage drop across it is added to the thermocouple E. M. F. The resistance 40 is utilized primarily for standardizing purposes and has a resistance value such that the voltage drop across it is equal to the voltage produced by the standard cell.

Slide-wire assembly 16 may include a cable drum 43 suitably rotated by the drive mechanism 25 through a cable 4. The term cable is used in a generic sense to cover similar a 'rangemcnts such as wire, violin string, rope, cable, etc. The cable drum 43 is provided with an adjustment for adjusting the diameter or circumference thereof. The cable drum 43 operates through an adjustable connection to move a contactor 45. The contactor 45 engages a slide-wire in the form of a resistance wire 45 which is space wound around an insulated core 41. Preferably the slide-wire 4E and the core 4? are formed of the same material such as manganin wire having substantially no temperature resistance coeiiicient. The contactor 45 also engages a collector bar in the form of a resistance wire 48 space wound around a core 49 and completely electrically conductive with respect thereto throughout their lengths. Preferably, the resistance wire 48 and the core 49 are formed of the same material, such as manganin wire, as the resistance wire 45 and core 41. The slide-wire assembly is provided with a terminal block 50 having terminals 5!, 52 and 53. The collector bar 48 is connected to the terminal 5i. One end of the resistance wire 46 is connected to the terminal 52 and one end of the core 41 is connected to the terminal 53, the other ends of the resistance wire E6 and the core 4'! being connected together. Connected across the slide-wire 46 and core 31, that is in shunt with the slidewire 46, is a resistance 5 also formed of manganin wire. The terminals 52 and 53 are connected to the ends of the resistance element 3| of mam i slide-wire resistance 46, the resistance 54 and the resistance 3| are all connected in parallel.

A three-position switch generally designated at 51- is utilized for performing three functions, namely, for connecting thepotentiometer circuit network for normal operation (run), for checking the system for errors (check), and for standardizing the potentiometer system (standardize). schematically, this switch 51 may comprise switch arms 58, 59, 68 and 6| electrically insulated from each other and all operated by a common operating member 62. The switch arms 58 and 59 oarrycontacts 63 and 64,"respectively. The switch arm 68 carries contacts 65 and 66 and the switch arm 6| carries contact 61. Contact 63 engages a contact 68 when the switch 51 is moved tothe standardize position and engages a contact 69-when the switch is moved to the check position. When the switch 51 is in the run position the contact 63 does not engage either contact 68 or 69. When the switch 51 is in run position the contact 64 engages both contacts 18' and 1N. When the switch is moved to the standardize position contact 64 disengages contact 1| and when the switch 51 is moved'to check position the contact 64 disengages contact 18*. Contact 65 engage contact 14 when the switch 51 is in run position. When the switch 51 is moved to standardize position contact 65 disengages contact 14 and contacts 65 and 66 engage contacts 12 and 13 respectively. When the switch 51 is moved to check position contact 65 remains in engagement with contact 14 and contact 66 engages contact 15. When the switch 51 is in either the run position or the check position contact 61 does not engage contact 16. Contact 61 engages contact 16 only when the switch 51 is moved to standardize position.

schematically the transformer I9 comprises a primary winding formed by two windings 18 and 1|. The adjacent ends of the windings 18 and 1| are connected together and to a terminal 12 and the other ends of the primary windings 18 and 1| are connected to terminals 13 and 14, respectively. The primary windings 18 and 1| are wound around a core structure 15 and also wound around the core structure is a secondary winding 16 connected between terminals 11 and 18. A shield 19 is provided between the primary windings 8 and 1| and the secondary windin 16. The windings and core structure of the transformer are housed in a casing 88; The core structure 15, the shield 19 and the casing 88 are connected to a terminal 8| which is in turn connected to ground.

schematically, the vibrator I8 may comprise a vibrating reed 83 for operating a contact 84 with respect to contacts 85 and 86. The contact 84 is connected to a terminal 81 and the contacts 85 and 86 are connected to terminals 88 and 89, respectively. The vibrating reed 83 is vibrated under the influence of a winding 98 connected between two terminals 9| and 92 and supplied with alternating current. A permanent magnet 93 associated with the vibrating reed is utilized for polarizing and synchronizing purposes. The mounting structure for the vibrating reed, contacts, operating winding and permanent magnet is connected to a terminal 94 which in turn is connected to ground. A suitable housing 95 encloses the mechanism of the vibrator I8.

For purposes of illustration it is assumed that the thermocouple I3 is an iron-constantan thermocouple. The lead I88 extending from the 6. iron element of the thermocouple to 'the positive cold junction terminal I8Iis an iron lead.

The lead I82 leading from the 'cons'tantan element of the thermocouple to the negative cold junction terminal I83 i made of constantan. The hot junction of the'thermocoupleis, therefore, in the furnace I8 and the cold junctionis at thecold junction terminals I8I'. enema. The positive cold junction terminal I8I is connected to thecontacts 16 and'15 of the switch 51 and is also'connected to a positive checking terminal I84. The negative cold junction terminal I83 is connected to a negative checking terminal I which in turn is connected by a jumper I86 to a negative checking terminal I81. The positive checking terminal I84 is connected to the terminal 12 at the junction of the two'primary windings 18 and 1| of the transformer I9 and the opposite ends of these primary windings 18 and 1| are connected, respectively, through terminals 13 and 14 and terminals 89 and 88' to theicontacts 88 and 85 of the vibrator I8. The vibrator contact 84 is connected through terminal 81 and the switch arm 68 to contacts 65 and 66. The contact 14 which normally engages the contact 651s connected to the terminal 5| and hence to the collector bar 48 of the slide-wire assembly I6. A cordingly the positive lead of the thermocouple is connected through the transformer and the vibrator to the collector bar 48 of the slidewire assembly |6 when the switch 51 is in the run position. The negative checking terminal I81 is connected through a resistance I88 to the contact 18 of the switch 51 and when the switch 51 1s inthe run position contact 18 is connected through contacts 64 and 1| to a point I 89 located between the compensating resistance element 39 and the resistance element 48 of the potentiometer network I5. I

The battery 33 operates to produce an E. M. F. between the contactor 45 of the slide-wire assembly I6 and the point I89 in the potentiometer network I5 which opposes the E. M. F. produced by the thermocouple I3 and the value of the battery E. M. F. produced between these points is regulated by the position of the contactor 45 along the slide-wire resistance 46. As the contactor 45 of the slide-wire assembly is moved in a clockwise direction the battery E. M. F. between the contactor 45 and the point I 89 is increased and as the contactor 45 is moved in a counter-clockwise direction the battery E. M. F. is decreased. When the potentiometer system is balanced the E, M. F. produced by the battery between the contactor 45 and the point I89 is equal and opposite to the E. M. F. produced by the thermocouple I3, and, therefore, no current flows in the thermocouple circuit.

Upon an increase in temperature within the furnace I8 the E. M. F. produced by thermocouple I3 becomes greater than the E. M. F. produced by the battery 33 between the contactor 45 and the point I89 which causes a D. C. current flow from the positive cold junction terminal |8I through the checking terminal I84 to the terminal 1 2 of the transformer I9 and thence through the primary winding 18, terminals 13 and 89 and contacts 86 and 84 to the terminal 81 of the vibrator I8 or through the primary winding 1|, terminals 14 and 88 and contacts 85 and 84 to the terminal 81. From the terminal 81 the D. C. current flows through contacts 65 and 14 of the switch 51, terminal 5| of the slide-wire assembly, collector bar 48, contactor 45, "slide-wire "resistance 46, potentiometer network I5 to I09, and then through contacts H 64 and ID of the switch 51, resistance I08, checking terminal I01, jumper I06, and checking terminal I05 to the negative cold junction terminal I03. This produces a D. C. current in one direction which is detected and amplified by the amplifier I! to operate the reversible motor 24 which through the drive mechanism 25, cable 44 and cable drum 43 moves the contact 45 in a clockwise direction to increase the battery E. M. F. between the contactor 45 and the point I09 to oppose equally the E. M. F. developed in the thermocouple. When this occurs the flow of D. C. current is stopped and the potentiometer system becomes balanced.

Upon a decrease in temperature within the furnace ID the E. M. F. produced by the thermocouple I3 becomes less than the E. M. F. produced between the contactor 45 and the point I99 by the battery 33. This causes a D. C. current to flow in the opposite direction from the contactor 45 through collector bar 48, terminal 5| of the slide-wire assembly I6, contacts I4 and 65 of the sun'tch 5?, terminal 8'! of the vibrator I8 and then through contacts 84 and 86, terminals 8-9 and T3 and primary winding I to the terminal I2 of the transformer !9 or through contacts 84 and 85, terminals 88 and 74 and primary winding II to the terminal I2. This D. 0. current then flows from the terminal I2 through positive checking terminal I04, positive cold junction terminal IOI, thermocouple I3, negative cold junction terminal I03, negative checking terminal I05, jumper I05, negative checking terminal I01, resistance I08, contacts 10*, 64 and H of the switch and point I09 through the potentiometer network I5 to the contactor 45 of the slide-wire assembly I6. This flow of D, C. current is detected and amplified by the amplifier I! to operate the reversible motor 24 in the opposite direction which in turn operates through thedrive mechanism 25, cable 44 and cable drum 4-3 to move the contactor 45 of the slide-wire assembly IS in a counter-clockwise direction. This decreases the E. M. F. produced by the battery 33 between the contactor 45 and the point I 09 to a value which equally opposes the E. M. F. produced by the thermocouple I3 whereupon the potentiometer system again becomes balanced and no D. C. current flows.

Acco dingly upon a change in temperature within the furnace $0 an unbalanced D. C. current is caused to flow in one direction or the other depending upon whether the furnace temperature increases or decreases. This unbalanced D. C. current flow is detected and amplified by the amplifier I? to operate the reversible motor 24 which in turn operates through the drive mechanism 25 and the slide-wire assembly I6 to reduce the unbalanced D. C. current flow to zero. The positions of the contactor 45 of the slidewire assembly I5 and of the indicating, recording and controlling parts mechanically coupled thereto represent the actual temperature value existing within the furnace I0.

Changes in temperature at the cold junction terminals I9! and I03 would normally tend to render the system inaccurate and in order to avoi this difficulty the cold junction compensation resistance 39 is included in the potentiometer network I5, this cold junction compensating resistance 39 being located in close proximity to the cold junction terminals WI and I03, so as to be similarly aifected by temperature conditions at this point. The cold junction compensation resistance element 39 is preferably made of copper having an accurate and uniform temperature resistance coefficient. This resistance element 39 is so connected into the potentiometer network that the voltage drop across it is added to the thermocouple E. M. F. whereby changes in temperature at the cold junction terminals IOI and I03 are entirely compensated for. A thermometer IIO may be utilized for measuring the actual temperature at the cold junction terminals IOI and I03 to aid in the calibration of the potentiometer system.

The slide-wire assembly I6 in addition to including the slide-wire 46 also includes the shunt resistance 54. During the manufacture of the slide-wire assembly the shunt resistance 54 is so calibrated with respect to the slide-wire resistance 45 that the total resistance of the s1idewire assembly is a fixed value. This resistance value is maintained the same for all slide-wire assemblies regardless of the rest of the potentiometer system. In other words the same slide-wire assembly I 5 may be interchangeably utilized for different ranges and ma also be interchangeably utilized in conjunction with various types of thermocouples, such as iron-constantan, copper-constantan and rare metal couples. All that is necessary to adapt the potentiometer system for use in various ranges and for use with various types of thermocouples is to change the calibrating resistances 30, 3| and 32.

In prior art potentiometer systems it has been necessary to closely calibrate the calibrating resistances 30, 3I and 32 and the cold junction resistance 39 to cause the instrument to operate accurately. Any small inaccuracy in the values of the resistance elements 30, 32 and 39 would cause the zero position of the instrument to vary. According to this invention an improved arrangement is provided. Here the resistances 30, 32 and 39 need only be constructed within reasonable limits instead of within the precise accurate limits which were previously required. To accurately align the zero position of the instrument, all that is necessary is to adjust the adjustable connection between the cable drum 43 and the contactor 45. This adjustable connection, therefore, provides an improved arrangement for obtaining zero adjustment. Any small inaccuracy in the value of the resistance element 3| would normally cause the range of the instrument to vary. According to this invention the resistance element 3I need only be constructed within reasonable limits instead of within precise limits and accurate adjustment of the range may be obtained by adjusting the diameter or circumference of the cable drum 43. This adjustment therefor provides an improved arrangement for obtaining range adjustment.

By having the resistances 3| and 54 connected in shunt with the slide-wire resistance 45, the effects of variations caused by variations in slidewire construction or wear of the slidewire is greatly decreased. According to the usual practice in the potentiometer art the impedance of the galvanometer or in this case the impedance of the input transformer I9 would be made substantially equal to the resistance of the rest of the potentiometer circuit including the thermocouple I3 and extension leads, slide-wire assembly I3 and potentiometer network I5. In an unbalanced potentiometer circuit any change in resistance in the remainder of the potentiometer circuit would materially affect the sensitivity of the galvanometer or the secondary voltage produced by the input transformer. Accordingto of the potentiometer circuit have practically no effect on the secondary voltage produced by the input transformer it.

In a practical and satisfactory embodiment, it is found that the system is substantially unaffected by changes in resistance when the impedance of each half of the primary of th transformer at the frequency of conversion is from to times the resistance .of the rest of the potentiometer circuit. By reason-of this selection of input transformer impedance, changes in thermocouple lead wire lengths, changes in contact resistance of the slidewire assembly and th vibrator contacts, different selections of thermocouple materials and extension leads and the introduction into the potentiometer circuit of additional resistance has practically no effect on the over-all sensitivity or operation of the complete system.

The'resistance H28 connectedto the negative checking terminal 16'. and the condenser I l 1 con nected across the thermocouple, are utilized for damping or anti-hunting operation and also for stray reduction purposes as will be pointed out more fully hereafter.

Power is supplied to the amplifier 11, the reversible motor 24 and a chart drive motor I2 by means of line wires I I5 and H6 leading from some source of alternating potential, not shown. 5

ture I23 in a synchronous fashion. As will be pointed out more fully hereafter the armature l23 operates the chart drive mechanism.

The amplifier I?! is provided with a transformer generally designated at F25 having a primary winding t26 and secondary windings 121', I26, I30 and till. A shield. I forstray elimination purposes is located between the primary and secondary windings and thisshleld along with the core and cover of the transformer are connected to ground. The secondary windings I36 and. [3| may be a single winding provided with a midtap M9. The primary winding 526 is provided with a tap 13. 2 near one end thereof. The opposite end, of the primary winding i28'is connected to the conductor. H9 and either the other end of the primary winding or the tap 32v is connected to the conductor I I9 through asingle pole double throw switch !33 and a fuse I34. The purpose of the switch I33. is to adapt: the transformer I25 for operation with either 110 volts supply or 125 volts supply. If the voltage supply is 125 volts the single pole-double throw switch I33 is in the position shown but if 110 volts are supplied the switch is thrownto the opposite position; The purposeof the switch 33 and the tap I32 is to'rnaintainthe number of volts per turn in the primary winding substantially constant regardless of whether 110 volts or 125 volts is supplied to the transformer.

The secondary winding lZ'lis utilized for sup-- 1 10 "plying alternating current to the operating winding of the vibrator 'l8' and to the filaments or heaters of the vacuum tubes 2%), 2|, 22 and 23. The connections are designated X and Y. The secondary winding I2! is preferably groundedat mid voltage to maintain the filaments or heaters at ground potential.

The vibrator it is essentially a polarized switching mechanism, the operating winding 93 and the permanent magnet 93 cooperating to vibrate the vibrating reed 83 at 60 cycles per second in synchronism with the 60 cycle alternating voltage supply. For purpose of illustration it is assumed that the contact 85 is engaged by the contact fi l-during the first half cycle of the alternating voltage supply when the voltage is positive and that the contact 85 is engaged by the contact 84 during the second half cyclewhen the voltage of the alternating voltage supply is negative. Accordingly the contacts 84 and 8 5 engage when the voltage of the alternating voltage supply is positive and the contacts 85' and '84 engage when 1e voltage of the alternating voltage supply is ne ative. When the vibrating reed t3 is stationary in the mid position both contact's'siand 86 are engaged by the contact 85 sothat when the vibrating'reed is operated the circuit through the primaries"!!! and H of the transformer l9'is never entirely interrupted.

When the potentiometer system is balanced no current flows in the potentiometer system and, therefore, operation of the vibrator I'8 has no 'effect. When'the temperature of the furnace It! increases the unbalanced D. C. current in the potentiometer system flows in a direction from the transformer 16 to the' vibrator l8 and, vice vers'a, when the temperaturewithin the furnace l0 decreasesthe unbalancedD. C. curren't'in the potentiometer system flows from the vibrator'l'8 to the transformer l9.

Referring toFigs. 2 to 6 curve A represents the phase of the supply voltage and it remains substantially thesam'e at all times. CurveB of Fig. 2 represents the potential of the terminal 1! when the potentiometer is balanced, it being substantially constant. Upon a temperature increase the potential of the terminall'l increases and decreases in phase with the supply voltage as illu's tr'ated by curves C of Figs. 3 and 4. It will be noted that the amplitude of curve C varies in accordance with the amount of potentiometer unbalance, it being greater for a large unbalance. Curves D of Figs. 5and 6repre'sent the potential of the terminal 11 when the temperature decreases and here it is seen that the potential increases and decreases degrees out of phase with the supply voltage. Here again the amplitude of curve D varies with'the amount of potentiometer unbalance. While the various wave forms are not necessarily true sine waves, they are so shown in Figs. 2'to 6 to more clearly illustrate the principles of this invention.

Assume now that the temperature in the furnace I0 increases. During the first half cycle of the alternating voltage supplied by the supply linesll5 and H6 the unbalanced D. 0. current flows from the terminal '72 through primary winding 10, terminal 13; terminal 89 and contacts 86 and 84' to the terminal 81' producing a current flow in the coil 10 to cause the terminal "to become more positive than theterminal 13. This current flow acts through the core structure 15 of the transformer l9 to induce a voltage in the secondary winding 16 causing the terrninal (1 tube positive with respect to the terminal 16 during the first half cycle. During the second half cycle of the alternating voltage supplied by the lines 2 I and H6 current flows from the terminal 12 through the primary winding 1|, terminals 14 and 86 and contacts 85 and 64 to the terminal 81 of the vibrator I8. This causes a current flow in the winding H to make the terminal 12 more positive than the terminal 14. This current fiow in the primary winding 1! acts through the core structure 15 to induce a voltage in the secondary winding 16 to cause the terminal 11 to become negative with respect to the terminal 18. Accordingly, under these conditions an alternating voltage is produced across the terminals 11 and I8 which is in phase with the alternating voltage of the supply lines I I5 and H6 as illustrated in Figs. 3 and 4 by curves C.

Assume now that the temperature in the furnace I 6 decreases. This causes an unbalanced D. C. current to fiow in the opposite direction from the vibrator I 8 to the transformer I9. During the first half cycle of voltage of the supply lines H5 and H6 current flows from the terminal 81 of the vibrator I8 through contacts 84 and 86, terminals 89 and 13 and primary winding 10 to the terminal 12. This causes a current to flow through the primary winding 10 which makes the potential of the terminal 13 positive with respect to the terminal 12. This current flow in the primary winding 10 operates in conjunction with the core structure to induce a voltage in the secondary winding 16 to cause the potentia1 of the terminal 11 to be negative with respect to the terminal I8 during the first half cycle. During the second half cycle of the alternating voltage supplied by the supply lines H5 and H6 current fiows from the terminal 81 of the vibrator I8 through contacts 84 and 85, terminals 88 and I4 and primary winding 1| to terminal 12. This causes a current flow in theprimary winding TI to cause the potential of the terminal 14 to be positive with respect to the terminal 12. This current flow through the primary winding II operates through the core structure 15 to induce a voltage in the secondary winding 16 to cause the potential of the terminal 11 to be positive with respect to the terminal 18 during the second half cycle. Accordingly when the temperature of the furnace I5 decreases the unbalanced D. C.

current flow in the potentiometer system produces an alternating E. M. F. across the output terminals 11 and 18 of transformer I9 which is 180 out of phase with the alternating voltage of the supply lines H5 and H6 as illustrated by curves D of Figs. 5 and 6.

Summing up when the potentiometer system is balanced there is no current flow through the primary windings 10 and II of the transformer I9 and hence the potential of the terminal 11 of the transformer I9 remains substantially constant. Upon an increase in furnace temperature the unbalanced D. C. current operates to cause the potential of the terminal 11 to increase and decrease in phase with the supply voltage. Conversely upon a decrease in furnace temperature the unbalanced D. C. current flow in the potentiometer circuit causes the potential of the terminal 11 to increase and decrease 180 out of phase with the voltage of the supply lines H5 and H6.

The amplifier I1 includes a continuous conductor I36 which is grounded at I31. The permanent magnet 93 and mounting structure of the vibrator I8 and the core structure I5, shield 16 and casing 86 of the transformer 13 are connected through this conductor I36 to ground. Likewise the secondary winding I21 of the transformer I25 is connected through the continuous conductor I36 to ground. The casing I38 of the amplifier I1 as well as the complete instrument casing is also connected to ground.

The vacuum tube 20 is a double triode tube of the indirect heated type and operates as two stages of amplification. The plate I40, cathode I42 and grid I43 are included in the first stage of amplification and the plate I44, the cathode I45 and the grid I45 are included in the second stage of amplification. The vacuum tube 2I is also a double triode tube, the plate I41, cathode I49 and grid I48 thereof acting as a third stage of amplification. The plate I55, cathode I5! and the grid I52 of the vacuum tube 2I act as a rectifier for supplying D. C. current to the plate circuits of the three stages of amplification. In this connection the grid I52 is tied to the cathode I5l.

One end of the secondary windin I28 of the transformer I25 is connected to ground through the continuous conductor I36 and the other end is connected to the plate I50 of the rectifier. The plate I 56, grid I52 and cathode I5I operate to produce a positive potential with respect to ground at the points I55, I56 and I51. The condenser I58 connected between the point I and the continuous conductor I36 to ground and the internal resistance of the tube 2I between the plate I50 and cathode I5I are utilized for filtering purposes to smooth out the ripple at the point I55.

- Likewise the resistance I59 and condenser I60 and the resistance I6I and condenser I62 operate to smooth out the ripples at the points I56 and I51. The points I55, I56 and I51 are, therefore, provided with one, two and three stages of filtering, respectively. Load resistances I65, I66 and I61 are connected into the plate circuits including the plates I40, I44 and I41, respectively.

The parallel arranged resistance I68 and condenser I69 located between the cathode I42 and the continuous conductor I36 to ground are utilized for biasing the grid I43 negative with respect to the cathode I42. The resistance I10 permits a flow of grid current between the grid I46 and the cathode I45 for limiting the extent to which the grid I46 may go positive with respect to the cathode I45. The resistance HI and contactor I12 connected to the grid I48 perform a dual function, namely, to limit the extent to which grid I 48 may go positive with respect to the cathode I49 and to vary the amount of signal impressed upon the grid I48 from the plate circuit of the second stage of amplification. The condenser I15 connected across the terminals 11 and 18 of the transformer I9 is provided for loading the secondary winding 16 of the transformer I9 to its ideal impedance at the line voltage frequency. The condenser I 16 connected between the plate I40 of the first stage of amplification and the grid I46 of the second stage of amplification is provided for eliminating the D. C. component of the plate circuit voltage on the grid I46 and for impressing the alternating component of the voltage produced across resistor I65 to the grid I 46. Likewise the condenser I11 connected between the plate I44 of the second stage of amplification and the grid I48 of the third stage of amplification is utilized for eliminating the D. C. component of the plate circuit voltage on the grid I48 and for impressing the alternating component of the voltage produced across resistor I65 to the grid I48.

The motor drive vacuum tubes 22 and 23 may 13 also be double triode tubes. The vacuum tube 22 includesa platel80, 'cathode'I8I 'and grid I82 andalso a plate I83, cathode I84 and grid I85.

.Thevacuum tube .23inc1udes a plate I86, cathode I81 and grid I88 Band also a plate I89, cathode I90 and grid. I'9I. vacuum tubes 22 and 23' are connected to the leftend of the transformer secondary windin I30 and the plates I83 and I89 are connected to the right end of the transformer secondary winding I3I. The'cathodes I8I,I84, I81 and I90 are connected together and through a'biasing resistance I93 and the continuous conductor I36 to ground. The grids I82, I85, I88 and I9I arecon- 'nected together and are also'connected-through The plates I80 and'IBB of the a condenser I 94 to the plate I41 of the third stage I.

of amplification. The condenser I94 is provided for eliminating the D. C. component of the'plate circuit voltage on the grids I82, I85, I88 and I9I, and also for impressing the alternating component of the voltage produced across the resistance I6! to these grids. The resistances I95 and I90 and the portion of the transformer secondary winding I28 to the right of point I9! permita flow of current between the grids I82, I85, I88 and I9I and the cathodes I8I, I84, I81 and I80,respectively, through the resistance I93 for limiting'the extent to which the grids may go positive with respect to their associated cathodes. The transformer secondary winding I28 and the resistance I96 also provide a source of alternating voltage to be applied to the grids I82. I85, I88 and I9I for a purposeto be pointed out more fully hereafter. The signal from the third stage of amplification is impressed simultaneously and equally on all of the control grids of the two vacuum tubes 22 and 23. From the above connections it will be noted that the vacuum tubes 22 and 23 are connected in parallel for'similar and simultaneous operatlon.

As illustrated, the motor 2:3 is a rotatin field motor including a stator 200 with four pole pieces physically spaced 90 apart and a squirrel cage rotor having interconnected conductor bars. Two of the opposite pole pieces are'provided with series connected power windings 202 and 203 and the other two opposite pole pieces are provided with series connected control windings 204 and 295. When the voltage and current through the motor control windings lag the voltage and current, respectively, in the power windings the rotor 20I rotates in a clockwise direction. When the voltage and current in the control windings lead the voltageand current, respectively, in the power windings the rotor 20I rotates in a counterclockwise direction. The motor 24 is preferably so constructed that the control winding 204 and 255 have'a high impedance to match the plate impedance of the motor drive vacuumtubes 22 and 23 when the motor is operating and also that the power windings 202 and 203 have a high impedance in orderto provide for efficient operation. Preferably, the control and power windings of the motor have a'high ratio of inductive reactance to resistance, for example, from 6 to l to 8 to 1; at 60 cycles. This provides formaximum power during running=with the least amount of heating and also provides 'a' low impedance path in't he control windings for anti-hunting purposes. In addition, because of this high ratio of inductive reactance to resistance in the power windings of the motor, the latter heats less when stalled than when running. I Electrical energy is supplied to the power windings 202 and 2030f the motor -24'from the conductor II8 through condenser 206, conductor 201, power winding's'202 and 203; and conductor 208 back to the conductor H9." The condenser-206 is so selected with respect to the power windings 2:12am 203 as to provide a substantially series 're'sonant circuit when the rotor Z 0I is rotating at approxim'a'telyfull speed. Due to this series resonant circuit the total impedance of the power winding circuit is substantially equal to the resistance of the power windings 202 and 203 and since-this resistance is relatively low, a large current flow through the power windings is made possible. This results in the production of maxi mumpower and torque by themotor. Dueto the series resonant circuit the current flow through the power windings 202 and 203 is in phase with thesupply voltage. However, the voltage across the power windings 202 and 203 leads the current by substantially 90, this being caused by the inductance of the windings. In Figs. 4 'and'd curve E shows the current flow in the motor power windings 202' and'203 to be substantially in phase with the supply voltage, and curve F shows the voltage across the power windings leading the current by substantially 90 when the rotor 20I is operating at approximately maximum speed. When the rotor 20! is operatin at substantially maximum speed, the inductance of the power windings 202 and 203 is' a maximum giving the above referred to series resonant condition. As

' the speed of rotation of the rotor'zill falls off the inductance of the power windings 202 and 203 decreases thereby upsetting to some extent the resonant condition, This causes a slight phase shift in the current through the power windings and the voltage across the power'windings, the voltage shifting slightly more than the current which reduces the power loss in the power windings. Further; the change from the resonant condition reduces the current flow through the power windings and due to the decrease in ind'uctance the voltage across the power windings likewise decreases. This also" reduces the power loss in the'windings. As a net result there is a subtantial'red'uction in heating of the power windin'g's when the rotor 20I is ata standstill as compared to a condition when the rotor is operating at substantially maidniu'm speed. Curves F and E of Fig. '2' show the voltage across and'the current through the power windings when the rotor is'at afst'a'ndstill and curves F and'E of Figs. 3 and5 show the same conditions when the rotor is oper ating' at an intermediate speed, The amplitudes of the currents and voltages and the phase shifts are set forth in Figs. 2 too.

Power is'supplied to the control windings 204 and 205 from the transformer secondary windings I30 and I3I through'theplate circuits of the motor drive vacuum tubes 22 and 23, resistance I93, continuous conductor I38, conductor 209, control windings 284 and 2:35 and conductor 2I0 to the center tap I29 of thetransf-ormer s'econdarywin'd ings I30 and I3I. A condenser 2H is connected in parallel with the controlwindings 204 and 205 and this condenser is' so selected as to provide a parallel resonant circuit during boththe stalled and running 'coriditions of the motor. This parallel'reson'ant circuit presents a relatively high external impedance and a relatively low internal cir tual resistance of the control windings 204 and 205, and since this resistance is relatively low, the impedance of the internal circuit is relatively low.

During the first half cycle of the alternating supply voltage the plate I80 of the motor drive tube 22 is positive with respect to the center point I29 of the transformer I25, and during the second half cycle the plate 500 is negative with respect to the center point 29 as illustrated by the curves G in Figs. 2 to 6. During the first half cycle of the alternating voltage the plate I83 of the motor drive vacuum tube 22 becomes negative with respect to the center point I29 and during the second half cycle the plate I83 becomes positive with respect to the point I29, as illustrated by curves H of Figs. 2 to 6. The voltage of the plate Isa: accordingly alternates in phase with the supply voltage and the voltage of the plate I83 alternates 180 degrees out of phase with the supply voltage. This relation always remains substantially the same.

On the assumption that the potentiometer system is balanced and, therefore, no unbalanced D. C. current flows in the potentiometer system, and on the further assumption that the point of connection of resistors I95 and I35 is connected directly to the continuous conductor I36, the potential of the terminal I? of the transformer IS remains substantially the same as the potential of the terminal 78 which is at ground potential as illustrated by curve B of Fig. 2. The potentials of the grids I43, 36 and I40 of the three stages of amplification remain substantially constant, and no signal is impressed upon the grids I82, I85, I83 and ISI of the motor drive vacuum tubes 22 and 23. Accordingly the potential f the grids of the motor drive vacuum tubes 22 and 23 remains substantially constant as illustrated by the curve I of Fig. 2.

Under these conditions current flows from the plate I80 to the cathode ISI when the plate is positive during the first half cycle, but current will not flow in this circuit during the second half cycle. During the first half cycle a pulse of current, therefore, passes from the plate I80 to the cathode I8I as illustrated by curve J in Fig. 2. During the first half cycle when the plate I83 is negative no current will flow from the plate I83 to the cathode I84, but during the second half cycle when the plate I83 is positive current will flow from the plate I83 to the cathode I84 as illustrated by the curve K of Fig. 2. Since the plate I85, cathode I3! and grid I83 of the vacuum tube 23 are connected in parallel with the plate I80, cathode I33 and grid I82 of tube 22, and t since the plate 539, cathode I99 and grid I9I of the vacuum tube 23 are connected in parallel with the plate !83, cathode I84 and grid I85 of tube 22, the vacuum tube 23 is connected in parallel with the vacuum tube 22 and operates in exactly the same manner as the vacuum tube 22.

Since the grids I82, I85, I83 and I9! are all connected together and since the potentials of these grids remain constant when the potentiometer circuit is balanced and the point of connection of resistors I95 and E98 is connected to the conductor I35, the vacuum tubes 22 and 23 produce pulses of current of equal magnitude during each half cycle of the supply voltage as is illustrated by the curves J and K of Fig. 2. The plate circuit of the vacuum tubes 22 and 23 include the parallel resonant circuit formed by the motor control windings 294 and 205 and the condenser 2II. The parallel resonant circuit offers a high external impedance which is substantially resistive in character, and accordingly a pulsating voltage drop in phase with the plate current is produced across the control windings 204 and 205 by the'flow of the pulsating plate current through the parallel resonant circuit.

The pulsating voltage across the control windings 204 and 205 produces a current through these windings which includes a D. C. component and an alternating current having a fundamental frequency of 120 cycles. Due to the inductance in the motor control windings 204 and 205 the current flow through the motor control windings is caused to lag the voltage across the windings by degrees of this cycle voltage or by 45 degrees of the 60 cycle supply voltage. The pulsating voltage across the motor control windings 204 and 205 is illustrated by the curve L of Fig. 2 and the alternating current flowing through the motor control windings 294 and 205 is illustrated by the curve M of Fig. 2. The condenser 2II connected in parallel with the motor control windings operates to maintain the D. C. component of the current fiow therethrough at a substantially steady value and also provides a low impedance path for the 120 cycle current flow in the motor control windings. Since the motor control windings have a low resistance the D. C. component of the current flowing through the motor control windings is relatively great while the amplitude of the alternating component of the current flowing through the motor control windings is relatively small because of the relatively high inductance of the motor control windings. The D. C. component of the current flowing through the motor control winding is illustrated by the curve N of Fig. 2.

Due to the relatively high D. C. current flowing through the motor control windings 204 and 205 when the potentiometer system is balanced, the core structure tends to become saturated and the inductive reactance of the motor control windings is relatively small. The condenser 2II is so selected with respect to this inductive reactance at 120 cycles that the condenser in parallel with the motor control windings forms a substantially parallel resonant circuit.

The relatively large D. C. current flowing through the motor control windings when the potentiometer system is balanced acts as a brake to prevent rotation of the rotor 2M. Rotation of the rotor causes the conductor bars thereof to cut flux produced by the D. C. current flowing through the motor control windings 204 and 205 and this produces a relatively heavy current in the rotor conductor bars which in turn quickly expends the force of rotation of the rotor. In other Words, the developing of a relatively heavy current in the rotor conductor bars by the rotation of the rotor acts to prevent rotation of the rotor, thereby providing an effective braking action. It is here noted that during the first half cycle of the supply voltage the A. C. component of the current flowing through the motor control windings 204 and 205 has a high peak and a low peak, the same being true for the second half cycle. The high peak and low peak of each half cycle cancel each other and, therefore, do not provide any turning efiort to the rotor 20I. This 120 cycle alternating current also acts the same as the D. C. current to provide braking. Rotation of the rotor also causes the conductor bars thereof to cut flux produced by the 120 cycle current flowing through the motor control windings 204 and 205 and this also 17 produces a relatively heavy current in the rotor conductor bars which in turn quickly expends the force of rotation of the rotor.

The amount of braking action is determined by the amount of D. C. current flowing through the motor control windings 204 and 205. The braking action may be increased or decreased by varying the amount of D. C. current flow. This may be done in various ways as by utilizing more motor drive vacuum tubes connected in parallel for supplying an increased amount of D. C. current, by selecting different types of tubes to provide the desired amount of D. C. current supplied to the motor control windings, or by varying the resistance value of the biasing resistance I93. The amount of braking action caused by the 120 cycle current may be similarly varied. By making the value of the biasing resistance I93 small the amount of braking action is increased and, vice versa, by making the resistance value larger the amount of braking action is decreased. If, however, the resistance value of the biasing resistance I93 is materially decreased the plate current through the motor drive vacuum tubes 22 and 23 will be increased thereby reducing the life of these vacuum tubes. Pref erably a compromise selection of the biasing resistance I93 is made to give the desired amount of braking with long life of the vacuum tubes and proper biasing of the grids of these vacuum tubes. By connecting the two tubes 22 and 23 in para! lel the life of the tubes is further increased.

From the above it is seen that when the potentiometer system is balanced the motor 24 remains stationary and any tendency for the mo- 1 tor to turn is rapidly reduced by the above outlined braking action.

Assume now that the temperature in the furnace I increases a relatively small amount, the potential of the terminal 11 of the transformer I9 increases and decreases in phase with the line voltage as illustrated by curve C of Fig. 3. This alternation of the potential of the terminal 'I'i forms a signal which is impressed on the grid I43 of the vacuum tube 29 and after being amplified by the vacuum tubes 20 and 2I the signal is impressed on the grids I82, I85, I88 and I9I of the motor drive vacuum tubes 22 and 23. The signal impressed on these latter grids is 180 degrees out of phase with the supply voltage as illustrated by the curve P in Fig. 3.

With this signal placed on the grids I82, I85, I88 and I9I of the motor drive vacuum tubes 22 and 23 the pulsating D. C. plate current during the first half cycle will be decreased since the potentials of the grids are more negative during the first half cycle and the pulsating D. C. plate current during the second half cycle will be increased since during the second half cycle the potentials of the grids are more positive. The values of the pulsating .D. C. plate current for the first and second half cycles are shown in Fig. 3 by curves Q and R, respectively. Accordingly, when the temperature of the furnace increases a small amount, the curve J of Fig. 2- decreases to curve Q of Fig. 3 and the curve K increases to curve R, the amount of decrease and increase varying in accordance with the amount of temperature increase in the furnace.

The decrease in magnitude of the pulsating plate current during the first half cycle decreases the magnitude of the pulsating voltage across the motor control windings during the first half cycle and the increase in the plate current during the second half cycle causes the voltagepulse across the motor control windings to increase during the second half cycle. Due to the cooperation of condenser 2H and motor control windings the voltage across the motor control windings decreases during the first half cycle at a faster rate than the voltage increases during the second half cycle. The curve S of Fig. 3 shows the voltage across the motor control windings during the first and second half cycles. From this curve it will be seen that the symmetrical pulsating voltage across the motor control windings is disappearing and that a 60 cycle component across the motor control windings is appearing. The high peak of the A. C. component of the voltage across the motor control windings appears during the second half cycle and leads the high peak of the voltage across the motor power windings by substantially degrees.

The alternating current component of the current fiowing through the motor control windings 204 and 205 is illustrated by the curve U of Fig. 3. Here it will be noted that the cycle component is diminishing and that a 60 cycle component is appearing. The high peak of the alternating current component of the current flowing through the motor control windings leads the peak of the current flowing through the motor power windings by substantially 90 degrees. At the same time the D. C. component of the current fiow through the motor control windings is decreasing whereby the braking action is decreasing. From the above it is seen that the current flow through and the voltage across the motor control windings 204 and 205 lead the current flow through and the voltage across the motor power windings 202 and 203. This causes rotation of the rotor 20I of the motor 24 in a counterclockwise direction. 'Ih'e speed-of rotation of the motor in the counter-clockwise direction depends upon the amplitude of the alternating voltage across and the alternating current flowing through the motor control windings and also depends upon the amount of D. C. current and the 120 cycle current flowing through the motor control windings. Whenthe temperature increase in the furnace is relatively small the motor will operate at a relatively slow speed.

Assume now that the temperature in the furnace increases a large amount the potential of the terminal TI of the transformer I9 will vary inaccordance with the curve C of Fig. 4. The potential of the terminal 11 increases and decreases in phase with the line voltage and the amplitude of alternation will be greater than that illustrated by the curve C of Fig. 3. The signal impressed upon the grids I82, I85, I88 and I9I of the motor drive tubes 22 and 23 is illustrated by the curve P of Fig. 4. Curve P of Fig. i is substantially the same as curve P of Fig. 3 except that the amplitude of the curve P of Fig. 4 is greater than that of the curve P of Fig. 3.

With this signal placed upon the grids I82, I85, I88 and I9I the pulsating D. C. plate current during th first half cycle will be decreased to zero sinc the potential of the grids is driven so far negative during the first half cycle that no current flows through the plate circuit during the first half cycle. The pulsating D. C. plate current during the second half cycle will be increased since during the second half cycle the potentials of the grids become still more positive. The values of the pulsating D. C. plate current for the first and second half cycles under these conditions are shown in Fig. 4 by the curves W and 

